The first patent application referenced above dealt with the use of pultruded composites generally with the construction of an electrical transmission tower. In the background of that disclosure is detailed the advantages of using pultruded composites as a replacement for steel in such a tower. These advantages result from physical, chemical, dielectric and electromagnetic properties of composite materials, which as used here refer to glass fibers woven into cloth which is drawn through a resin bath of polyester, vinylester, epoxy, or a thermoplastic such as polypropolene or polycarbonate, and then a die. A multiple-ply schedule is virtually always used.
The physical properties of composites that make it advantageous for use in electrical transmission towers are relatively high strength-to-weight ratio. However, it's low modulus of elasticity relative to steel requires that vertical support members and other structures be designed more creatively than is necessary with steel to mitigate the tendency of columns to buckle. Properly designed however, the tower structure is as strong or stronger than a tower of the same weight or cost made from steel.
Chemically, composites will resist corrosion and bacterial action indefinitely. They can be tailor-made to virtually any specifications depending on the application. Resistance to saltwater, ultraviolet light and virtually any destructive force can be designed into the composite.
The tower disclosed could be used for microwave, radio, or even windmill support, however, it is from the dielectric and electromagnetic perspectives that composites provide the greatest attraction in high voltage power line transmission towers. The high dielectric strength of glass composites and their low electrical conductivity make them much safer for repairmen, especially if the weather is inclement. When steel towers are maintained, there is a constant danger of flashover from a transmission wire to the steel tower structure. Wire grounding to the tower body may also be caused by a conductor being thrown across a tower structure in a storm. The steel construction aggravates many of the safety problems inherent in supporting high voltage power lines in the range of 115 kv. and higher.
An additional and unexpected advantage of composites over steel lies in the ability to bring the wires closer together due to the absence of the grounded conductive steel frame. Because the wires are out of phase with regard to the next closest wire or wires, the different phases will act to partially cancel out the electromagnetic field of adjacent wires and reduce the EMF at ground level. The closer the wires are together, the more cancellation of out-of-phase wires is exhibited.
By reducing EMF by bringing the conductors closer together, the tower can be made substantially smaller, with the highest conductors being substantially closer to the ground. This compaction reduces the widths of the right-of-way that must be purchased by the power authority to install the transmission towers. Currently, right-of-way purchase represents one of the major financial obstacles to electrical service expansion. This not only includes installation of new transmission lines but increasing the power capabilities of older lines. If a wider right-of-way must be purchased for a higher voltage transmission line due to ground level EMF regulations or just engineering standards, the cost may be prohibitive. However, using the tower described in this disclosure, of the same voltage class as the prior steel tower, a higher power transmission level can be achieved without expanding the right-of-way.
As discussed in one of the parent patents to this invention, the principal perennial problem with composites when used as structural members lies in the difficulty of forming joints. Steel is drilled, bolted and requires little imagination. Two steel beams or braces drilled and bolted together, or welded together, define a strong joint if done properly. The same techniques applied to composites will sever the continuous fibers which give the longitudinal fiber material its strength, and greatly weaken the structure at the joints. Therefore, steel is much simpler to use and currently structural engineers had steel construction emphasized in engineering school and in their professions and are generally entrenched in steel.
The need has thus presented itself to develop virtually innumerable new techniques for joining structural members of various sizes and configurations to withstand compression, sheer, and tension forces.
Although in the prototype stage substitution of composite structural members is more expensive than merely replicating steel configurations of the past, once the design has been perfected, pultrusion machines can run day and night with very little supervision, producing continuous elongated members having surprisingly complex cross-sections. Because the pultrusion process in its present state cannot create pultrusions of diverging or otherwise changing cross sectional configurations over the length of the piece, and thus cannot be strengthened at joint areas in the pultrusion process, connecting members together presents a challenge requiring considerable creativity.
The second-mentioned parent case to this invention tackled the problem of joining members of a large tower by the use of specially configured corner columns having slots which received specially designed cross members. That general concept is followed in this disclosure. The parent application used the low modulus of elasticity inherent in composite construction to create an internal slot or channel in the main support columns with deflecting detentes which received inserted ends of cross members having a mating detente configuration. When the cross members were inserted into the main slots of the columns, they locked in place. The column itself had internal partitions which yielded or expanded to admit the ends of the cross members.
In the instant disclosure, it has been determined that for the integrity and maximized strength of the columns, which will take the compressive loads of the tower, the columns should not define structural parts which yield to the insertion of cross members.